Halftone screen for use in the



April 1949- w. s. MARX, JR 2,468,680

HALFTONE SCREEN FOR USE IN THE MANUFACTURE OF PRINTING PLATES Filed July 2, 1948 2 Sheets-Sheet 1 w. s. MARX, JR 2,468,680 HALFTONE SCREEN FOR USE IN THE MANUFACTURE OF PRINTING PLATES 2 Sheets-Sheet 2 INFRA- RED ooh

April 26, 1949.

Filed July 2, 1948 00w 0% w 3 mm N o A R E W m C S l VU C M W m F. E

E T V P M L m m S E H mm m NR L M 5 fi w E 0 V NEW N m 1 00m mGF S E H v mo M fl v T I S S G v R N N N RD M F E FWT T Q. R v %R% V Nww M o- A CR 00 L Wmm 0mm l PE w mum 03 R. RMF.

O Q 0 O O 8 7 6 Z l 29 ww 2w2 m. momma m w o O O 8 2 .I l 20525524; .rzmumma WAVELENGTH Patented Apr. 26, 1949 HALFTONE SCREEN FOR USE IN THE MANU- FACTURE OF PRINTING PLATES Walter S. Marx, Jr., Santa. Barbara, Calif., as-

signor to Printing Arts Research Laboratories, Inc., Santa Barbara, Calif., a corporation of Delaware Application July 2, 1948, Serial No. 36,794

This invention relates to an improved halftone screen for use in the manufacture of printing plates, and more particularly to such a screen having light separating characteristics.

This application is a continuation-in-part of my prior application, Serial No. 582,751, filed March 14, 1945, now abandoned for halftone photoengraving screen, which latter application is a continuation-in-part of my prior application Serial No. 415,530, filed October 18, 1941, which has now become Letters Patent No. 2,373,489, issued April 10, 1945.

One of the objects of my invention is to provide an improved halftone screen for use in the manufacture of printing plates.

A further object of the invention is to provide an improved screen adapted to use in a preferred process whereby print-plates and negative elements therefor can readily be produced by photo-automatic means to give printed illustrations which are characterized by the absence of screen pattern in the highlights, by clean solid lines and type devoid of screen pattern, and shaded portions which are reproduced with the aid of the screen pattern as dots depending for size upon the shading of the original artists drawing. In other words, by the aid of this screen and the disclosed process, I am enabled to make reproductions of artists drawings in which the shaded portions are reproduced in substantially conventional halftone manner but in which lines and type are free from screen pattern and in which the highlights are completely free from screen pattern,

By a slight modification of my process, I am enabled to introduce a slight amount of screen pattern into the solid portions of the printing plate while at the same time preventing encroach ment of screen pattern upon the line and type elements.

An artists drawing comprises the following elements: First there is solid color" which will hereinafter be referred to by this term. Second, there are highlights which are free from pigment. Third, there are shaded portions which differ in degree of intensity of the pigment. Fourth, there is line. Line can be regarded as solid color of small width. Nevertheless, owing to the fact that it is often preferred in printing plates to have a small degree of screen pattern in solid color, and further owing to the fact that one of the principal objects of the present invention is to prevent the occurrence of screen pattern in line, it is necessary clearly to differentiate between these two elements of a 7 Claims. (Cl; 95-81) drawing. It will be understood that line is intended to include solid lines which the artist may use in making the drawing and also legends, numbers, and the like, which are referred to briefly as type.

In general terms, my invention involves the employment of two different kinds of light or, to be more exact, lighting of two ranges of wave length. The plates employed are sensitive to both kinds of light. My improved screen is provided with rulings or pattern composed of lines which are transparent or substantially transparent to one kind of light, While they are opaque to the other kind of light. While a screen of the type referred to may eifectively be used alone to accomplish the results of my invention, the emciency of performance may be improved to some degree by the addition of a filter having light transmitting characteristics similar to the screen lines. The drawing or material to be reproduced is either prepared or treated so that the shaded portions thereof are made opaque to that kind of light for which the screen pattern is transparent.

In actual practice it is preferred to employ visible light between the limits of substantially 4,000 and 7,000 angstrom units and ultra-violet light having an upper wave length limit of substantially 4,000 angstrom units as the two kinds of light and it is further preferred to make the screen with lines or pattern which are transparent to ultra-violet light and which are eifectively opaque or impervious to visible light, or more particularly to visible light having substantial actinic effect upon the photographic plates employed in the photoengraving and lithographic processes.

The employment of ultra-violet and visible light ranges is particularly advantageous in the preparation of the drawing, since the artist can, as will hereinafter be explained, employ pigments and colors which appear to the eye similarly to pigments and colors heretofore used. The pigments or colors may, however, as will hereinafter be described, contain material which renders them quite opaque to ultra-violet light. Consequently, the shaded portions will be quite opaque to ultra-violet light although to the eye of the artist, or indeed to the eye of any observer, they appear to be normal shaded drawings.

A The drawing prepared in this manner has solid color and line both of which are black or opaque to both visible and ultra-violet light. However, the shaded portions are substantially opaque to ultra-violet light. In the performance of the process, a negative is prepared with the aid of the screen and with the aid of both ultra-violet and visible light within the ranges specified. As has been indicated above, my improved process prevents the occurrence of screen pattern in the line, and unless additional steps are taken, the solid color will likewise be devoid of screen pattern. Since this is frequently considered objectionable for printing purposes, additional steps may be taken to provide screen pattern in the solid color while maintaining the line devoid of such pattern.

Other objects, advantages and capabilities of the invention will appear from the following specific description of preferred embodiments thereof taken in conjunction with the accompanying drawings in which:

Fig. 1 is a plan view of a screen embodying my invention;

Fig. 2 is a plan view of one of the plates from which my screen is fabricated;

Fig. 3 is a cross-section of the plate shown in Fig. 2;

Fig. 4 is a similar cross-section of the modified form of plate;

Fig. 5 is a fragment of a drawing whichis to be r pro u d;

Fig. 6 is a camera image of Fig. 5 as focused upon the plane of the screen;

Fig. '7 represents a developed negative of the positive shown in Fig. 5 after passage through the screen in the manner shown in Fig. 6;

Fig. 8 is a comparative developed negative similar to Fig, 7 but made with the aid of a conventional screen having lines opaque to all light;

Fig. 9 is an illustrative group of curves showing transmission spectra of screen elements in terms of percentage of light transmission and wave length; and

Fig. 10 is an illustrative group of'curves showin the light transmission characteristics of standard monocolor filters at various wave len ths of light.

It may be remarked that Figs. 5, 7 and 8 have been greatly enlarged from small portions of a drawing and negatives actually produced photographically. Indeed, all, of Figures 1 to 8 inclusive, of the drawing are greatly enlarged.

My improved screen which is shown in Fig. 1 may suitably consist of two sheets of glass which support between them lines t0 and l l of material Which is substantially opaque to the actinic portion of ordinary light and which is substantially transparent to ultra-violet light. The lines 0 H are arranged in intersecting relationship and they are preferably located at right angles to each other and in oblique relationship to the edges of the screen.

The two sheets of glass are secured together with the aid of Canada balsam or other suitable cement, or with the aid of any other suitable securing means, so that the lines 10 and II are in contact and in juxtaposed relationship between the two sheets of glass. The glass and cement, as may be understood, have a high degree of transparency (for example, over 90 percent) to all light within a range from below 3,400 angstrom units and extending above 6,000 angstrom units.

In Fig. 2, I have shown ,one sheet of glass having the lines I0 thereon. The lines [0 ll may be constituted by any suitable material which has the optical properties stated. Actually, I prefer to employ for the material of lines I0 and II a transparent substance or medium which is ilr preseated.v with a ab e ye which 4 imparts the desired light-filtering properties thereto.

In Fig. 8, the lines W are formed by dye-transparent material located in grooves l2 formed in the surface of the glass sheet. In Fig. 4:, the lines l0 are constituted by dyed-transparent material which is located on the surface of the glass sheet.

Referring to the embodiment of Fig. 3, the grooves l2 may be made with a diamond cutting device, the depth of the grooves being sufficient to allow for a deposit of a transparent resin or gelatinous material capable of transmitting near ultra-violet light, while absorbing visible actinic light.

Gelatin or polymerized methyl-methacrylate, commonly known as Lucite, or Plexiglas, or in deed any medium which is transparent to ultra- Violet and visible light in the ranges referred to and which will carry the dye employed, may be used as a filling medium for the grooves l2. These materials may be dyed with a pure grade of fuchsine and they then transmit light below 4,000 angstrom units and effectually absorb light between 4,000 and 6 ,000 angstrom units, as depicted by the curve 20 in Fig. 9. The collcdion wet plates, or the currently used types of halftone film, are not effectively sensitive to light of more than 6,000 angstrom units, as also indicated in Fig. 9. Consequently, the ruling or lines It! may transmit visible light of greater wave length than 6,000 angstrom units without. noticeable efiect.

The screen form shown in Fig. 4 can be .conveniently formed photographically from a master screen. The first step is to expose'and develop a photographic plate or cellulose acetate film so that the screen rulings appear as a silver image in the gelatin. A suitable dye such asfuchsine is substituted forthe reduced silver in the gelatin coating on the plate by methods known in the art. The remaining undyed gelatin may be either removed or allowed to remain between the dyed screen lines. In Fig. .4, the gelatin in which the silver was not reduced is shown removed, leaving only the lines [0 inthe form of dyed gelatin upon the surface of the glass plate or film. When film is used the resultant screen may be protected for use by mounting betweensheets of optically fiat glass.

It may here be noted that if gelatin is used for the base of the line ill, a water solution of fuchsine will dye it. In the case of the other plastic materials mentioned, a Water and alcohol solution of fuchsine may advantageously be employed.

It is .to be noted that while, I have successfully used fuchsine, it will-be understood that any other y or indeed any other material which transmits ultra-violet light and substantially opaque to actinic visible light, may be used instead of iuchsine. For example other dyes that may be used are acidified phenosafranine, alkali safranine, ortho phenanthroline, and colloidal silver.

It will of course be understood that materials other than fuchsine may be employed. Other materials which are efiecti vely transparent to ultra-violet light and effectively opaque to the actinic portionv of ordinary light include nitrosodimethylaniline, methyl violet, glass .or gelatin suspensions of nickel and cobalt salts, and suspensions of cobalt chloride, copper sulfate. acetamine scarlet B.

It will 'be, understood that thelines I50. and H may be of any desired number as in. conventional screens. Thus,thenumber of lines may be between50 and-300m thei-nch.

As may be observed by further reference to the curve 20 in Fig. 9, which curve represents substantially the light transmission characteristics of the lined portions of my preferred screen, those lined portions are substantially impervious (with less than percent transmission) to the actinic part of the visible light between wave lengths of 4,000 and 6,000 angstrom units. Those same lined portions of the screen, however, have transparency of over 70 percent to at least a portion of the ultra-violet light in the range of wave lengths between 3,300 and 4,000 angstrom units. This wide range of difierence in the transparencies of the lined areas to the ultra-violet and visible light-particularly the very low transmission of the visible light and the high degree of transparency to ultra-violet light in the actinic or photo-efiective range-serves as a useful light separating medium within the actinic range.

Also, as depicted in Fig. 9, the curve 2| shows that comparable light separating effects cannot be obtained by a combination of standard color filter elements having characteristics such as those indicated by curves 22, 23 and 24 in Fig. 10, or by the separate use of such filters. Likewise, curve 25 of Fig. 9 shows the lack of the desired color separating characteristics of a monochromous screen, such as magenta, which has been known in the prior art. Curves 2|, 22, 23 and 24 are also illustrative of known prior art screens and illustrate the distinguishing characteristics of my screen, as herein disclosed, from the color screens heretofore known in the art.

The artists drawing, a fragment of which is shown on enlarged scale in Fig. 5, may be prepared in the usual way, with the addition that the shaded portions are treated so as to be opaque or substantially impervious to ultra-violet light in a range of wave lengths between 3,300 and 4,000 angstrom units. This treatment should not afiect the appearance of the shaded portions to the eye so that the artist may visually obtain the desired shading which he desires to appear in the final reproduction. Furthermore, the shaded portions must maintain their relation to actinic visible light, within a range of wave lengths between 4,000 and 7,000 angstrom units, so that they may register selectively upon the negative in accordance with their density on the original drawing.

The most convenient way of making this original drawing having these optical requirements, is to mix with the pigment employed by the artist a material which is transparent to ordinary light and which is opaque to ultra-violet light. Thus, I may proceed, as far as the making of the original drawing is concerned, in the manner described in my Patent No. 2,191,939, issued February 27, 1940. As stated in that patent, I may use an ultra-violet absorbent substance, preferably of the fluorescent type, such as quinine bisulfate. The quinine bisulfate is dissolved in water and the solution is used as a solvent for the water-color or ink pigment used to make the drawing to be reproduced. The solution is quite colorless to the human eye and consequently the artist may proceed to make his drawing with the ink or pigment in the manner usual to him.

Actually I prefer to use the following solution:

Distilled water cc 1000 Quinine bisulfate "grams" 90 Sulphuric acid, C. P cc 10 Egg albumen, C. P grams The sulphuric acid amplifies the fluorescent and absorption properties. The egg albumen facilitates the smooth application of the pigment to the drawing. This solution is used instead of water as a medium for the artist's pigment. The pigment may be of any usual type; for example, it may be Windsor Newtons water-color lampblack which is representative of the type of material customarily used in making wash drawings.

It will of course be understood that I do not intend to limit my application to quinine bisulfate since any other material, transparent to ordinary light and opaque to ultra-violet light, may be employed. Other materials which may be substituted for quinine bisulfate to provide opacity to ultra-violet light include the following: anthracene, phenanthrene, various forms of naphthyl-amine disulphonic acid, beta methyl umbelliferone, and para-hydroxy-benzaldehyde.

The drawing is shown i'llustratively in Fig. 5,. areas l3 being highlights, area 14 being a lightshaded portion, area I5 being a heavier shaded portion, area It being a line, and areas I! being type considered as line. The drawing will also include relatively large areas of solid color (not shown) but the optical behavior, unless additional steps are taken, is the same for solid color as for line.

Apart from the light which will be discussed hereinafter, the procedure of making the negative with the aid of the artists drawing and employing my improved screen described above,may be effected in the normal manner. Thus, the negative may be placed in a photoengraving camera, my improved screen may be located in the camera at the proper distance ahead of the plate, and the drawing which is to be copied is placed. on a suitable support and illuminated by light. For the purposes of the present invention, the light must contain ultra-violet light and visible light. Suitably all the light is derived from a single source which supplies both kinds of light. However, the invention is not to be considered limited in this respect because two separate exposures may be made, one in which the positive: is illuminated by visible light, and another in which the positive is illuminated by ultra-violet light. The result is the same in both cases.

Such a source for a visible light and ultraviolet light may suitably be any of the ordinary white-flame carbon arcs which radiate amply in the ultra-violet region and also in the visible region. I may also use a lamp manufactured by the General Electric Company of Los Angeles, California, which consists of a high-intensity mercury-vapor argon-filled arc lamp in a double nonex or quartz nonex envelope. Light rich in ultra-violet is generated by electronic disturbances between two barium oxide coated cathode elements in the inner tube. The now popular mercury vapor lamps may also be used as an effective light source.

Considering the ultra-violet lightit will be understood that the picture reflects no ultraviolet light from the line portions l6 and I1, no ultra-violet light from shaded portions l4 and I5, and no light from solid color portions. The highlight portions I3 reflect the ultra-violet light and that ultra-violet light passes directly through all portions of the screen corresponding thereto.

Thus, referring to Fig. 6, it will be noted that the area of the screen including the lines It! and II, as Well as the open screen spaces between the lines around the letter F, is completely transparent to ultra-violet light. The same apx o ed: by; h l let ht a ne? y tions correspond completely. to the highlights l '3. e no d; a hs e= orti QIIB R I Qf ultra violet light alone is-sufilcient cc npletgly to expesehe; por of; he plate. o respond ng,

Considering now the. action, of v the :ultraviolet tive, corresponding. to these. areas. results solely frolnthe; visible light or. the visible component of the light.

Giving consideration tothevisible lighter the visible component of the light, the visible light striking. the. shadedareas? l4 and i5 is reflected thereby}. in amounts depending upon the. degree. QLshading. Thus, the portion Id being slightly shaded, reflects strongvisible light which strikes the screen; in the manner shown in Fig, 6,

Now, has. been described previously, the. screen is areal operative screen forvisible, light.

The. lines lfliof thescreen are opaque and have less than, 10 ,percent transparency to visible-light betweemwave lengthsof 4,000. and 6,000 angstrom;

units, and consequently the normal screen action occurs, in the .negative (Fig.- '7) shaded. portion l4,gives largeoverlapping dots of, reduced silver as shown in the upper le-fthandcorner of Fig. 7, while the heavily shaded portion l .5, gives small dotscorresponding to the openingsin ,thescreen asshown in the right-hand top corner of-Fig. 7. Thus, the. shaded por tions, print onthe negative (Fig. 7) in the usual 1174 .11

As has been previously, pointed out, the visible,- light reflected by the highlight porti0ns|3, does not aflect the corresponding portions of the neg-. ativelFig, 7) for the reason that the ultra-vim let light reflected from the highlight portions l3.

passes throughthe corresponding portions of the screen as if it were fully transparent, The visible light reflection from the highlight, portions l3 may tend to form. sornescreen pattern corresponding to the highlights l3 but thistendency is completely destroyed owing to the fact that the ultra-violet light corresponding to the high-. lights [-3 renders the corresponding portions of the negative (Fig. 7 completely black in the developed negative.

One outstanding effect of my invention is that thefline and solid'color are not ragged around the edges due to the enClfoachment on the lines, or solid color of clifiraction or lens action. of the screen. Thus, consider the line? l6 and rti u are. igh and nd f. his ne.

It willbe noted with reference toFig. 6 that this line passes across an opening in the. s9. t ep=.. I we cons de t s. openin it. wi

een hat a.certain...am unt. otvisih e light alls.

emp t ly xposed? s m aat h ti en he negat es... d loped er I are two o d; bla k: portions constituted by reduced silver whlch por- I ,thejhighlights l3 are completely devoidof; scr en: pattern; because the exposure from the The light type? ll. This may be donequ te roughly probrquag t. a ove; n b lch he;.1 .n '9 It would: b sume hat hi p ni would. act: ailens; and endi g prm .Qll' h m ative. F a acls otw i h w vrldi ie c oss h wh te. ma e: f n 6.: nhe;= esa i g his; end n o mino m or ance in my P fi$= for-two reasons: First, it may be; entirely avoided Whe e ondi on a t); y; mask n those; area th a clear p a ti Q erlaws et 9 d 1M r he. n nditypel, with a;dye -sn b.

stantially, matching the dye in,thqscreen-ru1ingi. thueblocking out most of theg-visible -light, while. fr fily'transmitting ultra-violet light; and; sec; ond; whateverslight dottingrnay appear at inf e;- d en n val o g edsg l Q ine near lie la, be s he v i l ht. alone. dm ted through the screen; opening o f; li igure 6iabo v ment einn ath r h p rt fz hg nenin r"- ferredtowhich is. obscured-by theline," l-fitis, insuflicient to activate much silverat; this point. This. dot-forming action or tendency.= '8- O L t" all assisted. by the ultra-violet light or the-ultra,- violetcomponent ofthelight. Ultra-violet light; finds the screen to be merelya-transparent plate so that the ultra-violet is notin. the least, diffracted.

Fig. Sisan enlargement, corresponding to Fig. 7 of a negative produced according'to standard practice, the degree of enlargement being the same. in both cases. Themannerin which the. fine line is lost and. the; sharp, contour. of, type is completel obliterated, will readily be seen from Fig. 8.

It is apparent. from the. previous description that solid color is reproducedin thenegative. inexactly; the same manner as the line. The marginal, definition of solid. color is. perfect andthereis a complete absence of screenpattern within the solid color. It isfrequently a,necessity for good printingthat solid, color? (and, not line. or type) should be. provided; with. screen pattern to a very slight degreetoprevent; subsequent deposit of excessive. inl; on solid color. areas when the; finished plate is printed.

. a Thi maybe-done by fiashingflthe.solid,color areas, to a very slight extent. This flashing has no effect uponthe shaded, portionsf because it merely. results in the. formation of'small or .incip lent. dots. which are completely masked-by the;

; larger dots as a result of normal exposureoi the,

shaded portions to the visible light or. the visible component of the light. Ordinarily, hOWr", ever, it,.is.important and desirable-that the sharp. definition ofv the line and the .absence. there-,

.fromof any encroachment of screen pattern,

should be very carefully preserved.

Flashing maybe effected simply; by placing a. layer of. tissue paper over'the drawing of Fig." 5, and then. drawing on the tissue paperin black, matter corresponding tothe line? [6 and. the.

videdi the. roughly applied color. does not overlie. solid color portions of considerable size and inwhich it is desired tocreate screenpattern. The

, plate is then given a very small exposure with the lens stopped down and withvisible light only. The degree of this exposure is merely su fiicient to givea certain amount; ofv screen pattern in: thelarge solid colorportions. The tissue paper is then removed and the exposure of the negative. is. proceeded with in the manner described above. Of course, it; isnot to be; deducedfrom the foregoing that it is necessary to do the flashing before the exposure since it wille clearly p rent -th se 'SKil dimthe. ar

that the order of exposure is quite immaterial.

Although my invention has been described in connection with specific details of the embodiments thereof, it must be understood that it is not intended to be limited thereto except in so far as set forth in the accompanying claims.

Having thus described my invention, I declare that what I claim is:

1. A halftone screen adapted to use in a photographic reproduction process with an emulsion having an upper limit of light sensitivity of approximately 6,000 angstrom units and comprising, in combination, a sheet of material having transparency of over 80% to all light in the visible and ultra-violet ranges between 3,400 and 7,000 angstrom units, said sheet having lines thereon disposed in transversely intersecting relationship, and said lines being characterized by their substantially complete absorption of all light within the range of 4,000 to 6,000 angstrom units and a transparency which ranges to more than 70% to the ultra-violet light between 3,400 and 4,000 angstrom units.

2. A halftone screen for use in a photo-reproduction process wherein a photographically actinic light includes both visible and ultra-violet rays in a range of wave lengths between 3,400 and 6,000 angstrom units, and comprising, in combination, sheet material having a transparency of over 80% to both the visible light between wave lengths of 4,000 to 6,000 angstrom units and. ultra-violet rays having wave lengths between 3,400 and 4,000 angstrom units, a plurality of lines in intersecting relationship carried by said sheet material, and said lines being of substantially uniform density and having a transparency of over 70% to a large portion of the ultra-violet light rays between wave lengths of 3,400 and 4,000 angstrom units and having less than 10% transparency to substantially all visible light rays having wave lengths between 4,000 and. 6,000 angstrom units.

3. A halftone screen comprising two plates of material over 80% transparent to ultra-violet light within the wave length range of 3,400 to 4,000 angstrom units and visible light within the wave length range of 4,000 to 7,000 angstrom units and having faces thereof arranged in juxtaposition, material defining lines of uniform density on the adjacent faces of said plates, said lines being in intersecting relationship, said line defining material being as much as 70% transparent to ultra-violet light within said range and substantially impervious to substantially all visible light having wave lengths between 4,000 and 6,000 angstrom units.

4. A halftone screen comprising two plates of transparent material having faces thereof arranged in juxtaposition, gelatinous material defining lines on the adjacent surfaces of said 10 plates, said lines being in intersecting relationship, said gelatinous material being dyed with a water solution of fuchsine to render it substantially impervious to visible photographically actinic light including a plurality of colors while remaining transparent to ultra-violet light.

5. A halftone screen adapted to use in a photographic reproduction process with an emulsion having an upper limit of light sensitivity of approximately 6,000 angstrom units and comprising, in combination, a sheet of material transparent to all light in the visible and ultra-violet ranges between 3,400 and 6,000 angstrom units, said sheet having lines thereon disposed in transversely intersecting relationship, and said lines being characterized by having less than 5 percent transparency to all light within the range of 4,200 to 6,000 angstrom units and over percent transparency to ultra-violet light between 3,400 and 4,000 angstrom units.

6. A halftone screen for use in photo-reproduction processes wherein the photographically actinic light includes both visible and. ultraviolet rays and. comprising, in combination, sheet material transparent to both the visible and ultra-violet rays between 8,400 and 6,000 angstrom units, a plurality of lines in intersecting relationship carried by said sheet material, and said lines being characterized by a single and sharply defined difference of transparency occurring at substantially 4,000 angstrom units between the ultra-violet light rays and the visible light rays up to 6,000 angstrom units, the transparency to the ultra-violet light rays being as high as 70% and a multiple of over seven times the transparency to said visible light rays.

7. A halftone screen for the production of combination line-and-halftone negatives from copy treated to absorb ultra-violet light in the halftone areas while reflecting ultra-violet light from the line and background and white areas, said screen comprising intersecting lines which transmit as much as 70% ultra-violet light having wave lengths between 3,400 and 4,000 angstrom units and are efiectively impervious to all visible light between 4,000 and 6,000 angstrom units.

WALTER S. MARX, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,162,162 Hatt Nov. 30, 1915 1,988,891 Gaspar Jan. 22, 1935 2,304,988 Yule Dec. 15, 1942 

